1. Field of the Invention
This invention relates to a system for cancelling echo signals which occur during transmission of digital signals in a duplex channel system over a two-wire line. The system includes a transmitter for emitting digital signals to the two-wire line, a receiver that picks up digital signals from the two-wire line, and an adaptive compensation circuit for producing compensation signals which compensate for the echo signals which are picked up by the receiver in addition to the received digital signals. The compensation circuit supplies the compensation signals to the receiver in response to transmitted data and echo-compensated received digital signals from said receiver.
2. Description of the prior Art
One form of echo cancelling system is known from, for example, "Philips Technische Rundschau", 39, 1980/81, No. 3, Pgs. 82-98. In this system, the compensation circuit is arranged as an adaptive transversal filter, and is especially suitable for compensation of echo signals that are linearly related to the digital signals to be transmitted. This type of compensation requires considerable circuit complexity.
Examples of compensation circuits for echo signals which are nonlinearly related to the digital signals to be transmitted are described in "IEEE Transactions on Communications", VOL. COM-30, No. Nov. 11, 1982, Pgs. 2421-2433. A first example of a compensation circuit works in accordance with a so called memory compensation principle. According to this principle, compensation signals corresponding to all of possible combinations of N successive digital signal states which may be transmitted are stored in a memory. The memory is addressed upon each transmission of a signal state, in order to generate the corresponding compensation signal corresponding to the current signal state and the (N-1) previously transmitted signal states. With such a compensation circuit, nonlinearities in the echo signals may be compensated. However, the response time of the compensation circuit depends upon the total number of combinations of said N successive signal states that must be considered.
A second example of a compensation circuit is based upon a conventional linear echo compensator, including, for example, a transversal filter, but also including additional compensating stages corresponding to the nonlinearities in the echo signals that must be considered. Such a compensation circuit has a more favorable response than the memory-based compensation circuit. However, the response of the section of the compensation circuit compensating for the linear echo signal portion of the received signal may be affected by the nonlinearities in the echo signal. For example, in a case, in which the main component of an echo signal is a linear echo, the minor nonlinearities still affect the response of the compensation circuit.
A compensation circuit is desired that can effectively compensate for echo signals with both a linear and a nonlinear signal component and provide a favorable response characteristic.
An example of a nonlinear echo compensation circuit consists of a series connection of a linear echo compensator and a nonlinear echo compensator. Because of this separation, the response of the linear echo compensator is largely independent of the nonlinearities in echo signals that must be considered.